The present invention was disclosed to the Patent Office in a technical disclosure filed with the Patent Office on Oct. 18, 1990, and assigned number 265,544.
Certain heat exchangers and furnaces operate at extremely high temperatures, and the efficient operation of the heat exchanger or furnace often requires that certain portions of the apparatus operate within relatively narrow temperature ranges. The operation of such furnaces and heat exchangers, therefore, requires accurate monitoring of various surfaces within the exchanger or the furnace. Generally, the temperature of the surface to be measured is different from the temperature of the surrounding gases which may be used to heat the surface, or are in turn heated by the surface the temperature of which is to be measured. In any case, the device used to measure the temperature of a surface must be isolated so as to read the temperature of the surface, and not the temperature of the surrounding gases.
Where thermal energy is transmitted through gases to heat a surface, the isolating enclosure which protects the device from measuring the temperature of the surrounding gases also influences the temperature of the surface which is to be monitored. The area of the outer surface of the enclosure is greater than the area of the monitored surface which is covered by the enclosure for the temperature measuring device. The increased area of the enclosure causes what is known as a "fin effect" whereby a greater amount of energy is conducted from the enclosure to the monitored surface in the proximity of the measuring device. The fin effect is increased by the metal sheath which encloses the lead wires extending from the thermocouple. On the other hand, where a surface is being heated by radiant energy, the presence of an enclosure protecting a temperature measuring device will intercept radiant energy before it reaches the surface causing what is known as a "shadow effect" which results in the underlying surface being at a lower temperature than the surrounding surface. To minimize the temperature distortions caused by the fin effect and the shadow effect which is created by an enclosure, the enclosure of a temperature measuring device should be made as small as possible.
The leads extending to the thermocouple must be electrically insulated from the surrounding enclosure, but the electrical insulation surrounding the leads is also a thermal insulator. It is also desirable that thermal energy from the monitored surface be readily transferred to the thermocouple and, therefore, that there be a minimum of electrical insulation, or no electrical insulation between the thermocouple and the monitored surface.
Existing thermocouples used in high temperature heat exchangers are formed with the hot junction welded to and thermally connected to the sheath enclosure of a thermocouple cable. In such devices the thermocouple is spaced from the surface to which the device is attached. The accuracy of the thermocouple will be influenced by the spacing from the exchanger wall, and by heat which is conducted along the sheathing surrounding the cable.
In my prior Pat. No. 4,971,452, I disclosed the use of an RTD assembly for measuring the temperature of hot surfaces and enclosures for mounting such assemblies. Although RTD assemblies are useful for measuring the temperature of hot surfaces and the like, such RTD assemblies do not currently function at very high temperatures. Where surfaces are maintained at very high temperatures, it is desirable that the temperature measuring device applied to such surfaces be a thermocouple. Accordingly, there is a need for an improved thermocouple which minimizes the fin effect and shadow effect caused by the isolating housing used to mount the thermocouple against the monitored surface.
There is also a need to improve the accuracy of the temperature readings received from such thermocouples by reducing or eliminating thermal insulation between the thermocouple and the surface being monitored.